Vacuum gauge having separate electron collecting and electron accelerating electrodes



VACUUM GAUGE HAVING SEPARATE ELECTRON COLLECTING AND J 968- w. A. LLOYDETAL 3,387,175

ELECTRON ACCELERATING ELECTRODES Filed March 5, 1965 INVENTORS WILLIAMA. LLOYD ROB RT JEPSEN BY a4...

v ORNEY United States Patent 3,387,175 VACUUM GAUGE HAVING SEPARATEELECTRON COLLECTING AND ELECTRON ACCELERATING ELECTRODES William A.Lloyd, San Jose, and Robert L. Jepsen, Los Altos, Calif., assignors toVarian Associates, Palo Alto, Calif., a corporation of California FiledMar. 5, 1965, Ser. No. 437,456 11 Claims. (Cl. 315-108) ABSTRACT OF THEDISCLOSURE The effective X-ray limit in low pressure ion gauges issubstantially reduced if not entirely eliminated in a gauge in whichelectron collection is effected at potentials too low for anyappreciable X-ray emission caused by electron bombardment. In thedisclosed gauge, electrons are emitted from a ribbon filament coaxiallydisposed about the lower end of an elongated high voltage acceleratinganode. As the electrons accelerate toward the high voltage anode, theyare caused to miss and spiral about the high voltage anode by a magneticfield of about 600 gauss until they reach and are collected by a lowvoltage electron collector surrounding and coaxial with the upper end ofthe high voltage anode. The potential on said low voltage electroncollector is insufficient for appreciable X-ray emission by electronbombardment. In

v addition an ion collector is so arranged that if a few X-rays areemitted, the secondary electrons they liberate from the ion collectorwill be returned to the ion collector by the magnetic field.

With the development of the Bayard-Alpert gauge (21 Review of ScientificInstruments 571, 1950), scientists were presented with a laboratoryinstrument capable of pressure measurement two orders of magnitude lowerthan theretofore known. Attempts at achieving still lower pressures havebeen hampered through failure to construct a gauge whose X-ray limit isless than 2 l0- torr, the accepted limit of the Bayard-Alpert gaugetoday. This invention relates to an improved ionization gauge whoseeffective X-ray limit has been substantially lowered, and in fact hasnot yet been determined.

Present day ionization gauges normally include: a relatively smallsurface area ion collector, for example, a thin wire; a highlypositively charged grid surrounding the collector for both acceleratingand eventually collecting the electrons, and a filamentary cathodedisposed outside the grid for emitting the electrons. It has been knownthat the electrons will eventually strike the grid and with sufiicientenergy to give rise to X-ray production. The ion collector, despite itssmall size, will still intercept a portion of these X-r-ays, which leadsto photoelectric emission of electrons from the ion collector surface.As is well known in the art, the arrival of positive ions at the ioncollector surface is indistinguishable from the departure ofphotoelectrons therefrom in the ion collector measuring circuit, therebyestablishing an effective lower limit to the pressure measurable. Bymeasuring the X-ray current in the gauge, pressures somewhat below theX-ray limit can be inferred (by subtracting the X-ray cur rent from thetotal current), but the precision with which the pressure can bedetermined is seriously impaired by the presence of this X-ray current.

It is the principal object of the present invention to provide animproved ionization gauge having a substantially reduced X-ray limit.

Briefly stated, in accordance with one teaching of the present inventionthere is disclosed an ionization gauge in which the functions ofaccelerating and collecting the 3,387,175 Patented June 4, 1968electrons are, in effect, separated by the provision of a high voltageelectrode for accelerating electrons and a low voltage electrode forcollecting electrons, thereby substantially reducing the production ofX-rays. In accordance with another aspect of the present invention,magnetic means are provided, being so arranged with respect to theelectrodes of the gauge that electrons are compelled to move in such away as to miss the electron accelerator electrode, but have a highprobability of striking and being collected at the low voltage electroncollecting electrode after having pursued a greatly lengthened electronpath. Because of the low energies with which the electrons arecollected, the rate of X-ray generation is much reduced. At the sametime most of the photoelectrons which may be produced by any residualX-rays striking the ion collector surface may be constrained by themagnetic field in such a way as to follow paths returning to the ioncollector, consequently reducing further the X-ray effect.

One feature of the present invention is the provision in an ionizationgauge of a high voltage electrode for accelerating electrons, a lowvoltage electrode for collecting electrons, and magnetic means soarranged that electrons are compelled to move in such a way as to justmiss the electron accelerator electrode, but have a high probability ofstriking the electron collecting electrode after having pursued agreatly lengthened electron path.

Another feature of the present invention is the provision of anionization gauge of the above type wherein the ion collector is soarranged that most photoelectrons which may be produced by any residualX-rays striking the ion collector surface are constrained by themagnetic field in such a way as to follow paths returning to the ioncollector.

These and other objects and features of the present invention and afurther understanding may be had by referring to the followingdescription and claims taken in conjunction with the following drawingsin which:

FIG. 1 is a fragmentary cross-sectional view of a novel gaugeconstructed in accordance with the teachings of the present invention,shown connected to a system whose pressure is being measured, andincluding the associated circuitry in schematic;

FIG. 2 is a cross-sectional view taken along the lines 2--2 of FIG. 1;and

FIG. 3 is a fragmentary cross-sectional view of another embodiment ofthe electron accelerator of the present invention.

Referring now to FIGS. 1 and 2, there is shown an ionization gauge 11constructed in accordance with the teachings of the present invention.The gauge 11 includes a primary source of electrons 12, an electronaccelerator 13, an electron collector 14, an ion collector 15 andmagnetic means 16.

The source of electrons 12 may be of the filamentary thermionic typemade of, for example, thoria-coated iridium in the shape of a thin,cylindrical ring coaxially supported at the base of gauge 11 on heaterlead-in wires 17, 18 about the lower end of the electron accelerator 13.Electrode 12 acts as a source of primary electrons which collide withgas molecules to form positive ions.

Electron accelerator 13 is an elongated metal member made of, forexample, stainless steel in the shape of a thin rod of relatively smallsurface area supported on a lead-in wire 19 along the axis of ioncollector 15. Electrode 13 is maintained at a high positive potential soas to accelerate electrons but is so positioned, as will be explainedfurther, that it intercepts practically none of these electrons.

The electron collector 14 is a short cylindrical member as of stainlesssteel disposed about one end of the electron accelerator 13 andsupported on a lead-in wire 20. Electrode 14 is so arranged as tointercept and collect nearly all of the electrons and maintained at alow enough positive potential that striking electrons will not cause X-rays to be emitted from the electron collector surface.

The ion collector 15 is an elongated, cylindrical member made of, forexample, stainless steel and forms a portion of the outer wall of thegauge 11. The collector 15 is typically at or near ground potential andserves to collect atoms or molecules ionized positively as a result ofcollisions with electrons.

A glass or ceramic header assembly 21 through which the lead-in wirespass in electrically isolated and vacuumtight manner is sealed to thebase of collector 15. A glass or ceramic ring 22 sandwiched between thetop of collector 15 and a metallic ring 23 serves to maintain vacuumintegrity of gauge 11 and electrically isolates collector 15 from ring23. Ring 23 is welded to a vacuum flange 24 and may be removablyconnected in vacuum-tight manner to a mating flange 25 welded to thewall 26 of a system whose pressure is being measured upon compression ofa soft metal gasket 27 as, for example, by tightening circumferentiallyarranged bolts 28. Thus, gauge 11 is illustrated as an evacuableappendage to the wall 26 of the system whose pressure is being measured.

The magnet means 16 comprises an annular cylindrical member which may beeither a permanent magnet or an electromagnetic coil slidably mountedabout and insulated from, by means of an insulating sleeve 29, the ioncollector 15. Ceramic header 21, ring 22 and sleeve 29 must be of veryhigh resistance so as to prevent development of leakage currentsthereacross.

In accordance with one aspect of the present invention the magneticmeans 16 is so arranged with respect to the electrodes of the gauge 11that electrons emitted from the source 12 are compelled to move inmodified cycloidal paths just missing the electron accelerator 13 asthey proceed axially upward toward the electron collector 14.

It has been observed that there exists a residual current to the ioncollector 15 caused by ion emission from the filament 12. Ion collector15 is electrostatically shielded from the filament 12 by the provisionof ion suppressor means comprising a cylindrical member'30, preferablystainless steel, supported on lead-in wire 31.

In operation, the ion collector 15 is essentially at ground, the ioncurrent being measured by any suitable means 32 capable of measuringvery small currents. The filament 12 is maintained at +l-40 volts withrespect to ground by means of a power supply 33, the electronaccelerator 13 is maintained at +175-500 volts with respect to ground bymeans of a power supply 34, the electron collector 14 at +-30 volts withrespect to the filament 12 by means of a power supply 35, and the ionsuppressor means 30 at 20 volts with respect to the filament 12 by meansof a power supply 36. Power is supplied for heating filament 12 by meansof a power supply 37, typically 2-3 volts and 3-4 amperes. The magneticmeans 1 6 provides a magnetic field intensity of 600-800 gauss withinthe electrode region of the gauge 11.

Electrons are emitted from the filament 12 and accelerated toward theelectron collector 14 principally due to the potential on the electronaccelerator 13. In other embodiments, as illustrated in FIG. 3, forexample, the electron accelerator 13 may be so constructed as to furtheraid in drawing olf electrons from the filament 12, for example, by theprovision of an outwardly extending disc-like member 38. Beforecollection, however, a certain number of the electrons will collide Withgas atoms and molecules, thereby forming positive ions. These positiveions are collected at the ion collector 15, causing current flow throughmeans 32, the number collected being an index to molecular density,i.e., pressure. The magnetic means 16 is so arranged with respect to theelectrodes of the gauge that electrons are compelled to move in modifiedcycloidal paths just missing the electron accelerator electrode 13 asthey proceed axially upward, but have a high probability of striking andbeing collected at the electrode 14, after having pursued a greatlylengthened electron path. At the same time any photoelectrons which maybe produced by X-rays striking the ion collector surface are constrainedto return to the ion collector, consequently greatly reducing X-rayeffect.

In an actual embodiment and using potentials in the above specifiedranges, an Alnico VIII magnet providing a field intensity of 670 gauss,a collector 1% inches in diameter and 1% inches long, an electronaccelerator with a rod /s inch in diameter and a filament inch indiameter set in 0.4 inch from the end of the electron accelerator rodand an electron collector of one inch diameter and 0.25 inch long wereused. Several advantages were realized from the above construction. Forexample, there was increased sensitivity due to the proximity of theelectron cloud to the ion collector and the increased electron pathlength.

Since many changes can be made in the above construction and manyapparently widely dilferent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An ionization vacuum gauge for determining pressure within a givenspace comprising: an elongated relatively small surface area electronaccelerator; an electron source positioned adjacent one end of saidelongated electron accelerator; an electron collector positionedadjacent the opposite end of said elongated electron accelerator andinsulated therefrom; an ion collector spaced apart from said electronaccelerator, and magnet means to produce a magnetic field having linesof force parallel with the axis of said electron accelerator.

2. A gauge according to claim 1 wherein said magnet means is so arrangedthat photoelectrons produced by X-rays striking said ion collector areconstrained to return to said ion collector.

3. The gauge according to claim 2 wherein said electron accelerator andsaid magnet means cause the electrons to pursue modified cycloidal pathsabout said accelerator as they proceed in the direction of said electroncollector.

4. A gauge according to claim 1 wherein said electron source and saidelectron collector are each cylindrically shaped electrodes coaxial withsaid elongated electron accelerator.

5. A gauge according to claim 4 including means for applying a firstpotential to said electron accelerator; and means for applying a secondpotential to said electron collector, said second potential being lessthan said first potential and insufficient to cause appreciable X-rayemission by electron bombardment.

6. The gauge according to claim 4 including means coaxial with saidsource of electrons for electrostatically shielding said source ofelectrons from said ion collector.

7. An ionization vacuum gauge for determining pressure within a givenspace comprising: four spaced-apart electrodes forming an electroderegion including, a tubular ion collector, an elongated, relativelysmall surface area electron accelerator extending coaxially within saidion collector, a source of electrons positioned within said ioncollector and about one end of said electron accelerator and acylindrical electron collector disposed within said ion collector andabout the opposite end of said electron accelerator; and, means toproduce a magnetic field having an axis coinciding with the axes of saidion collector and said electron accelerator.

8. The gauge according to claim 7 including a discshaped memberextending outwardly from said electron accelerator near said source ofelectrons.

9. An ionization vacuum gauge for determining pressure within a givenspace comprising: an electron source, an electron accelerator, anelectron collector insulated from said electron accelerator, and an ioncollector; means applying a high accelerating potential to said electronaccelerator, and means applying to said electron collector a collectingpotential substantially lower than said accelerating potential.

10. A gauge as claimed in claim 9 in which said accelerating potentialis 175 to 500 volts above ground and said collecting potential is 25-70volts above ground.

11. A gauge as claimed in claim 10 further comprising means applying15-40 volts above ground to said electron source.

References Cited UNITED STATES PATENTS Re. 25,369 4/1963 Redhead 3137 X3,193,724 7/1965 Klopfer 315-108 X 3,319,117 5/1967 Wheeler 315-108 X 10JAMES W. LAWRENCE, Primary Examiner.

S. A. SCHNEEBERGER, Assistant Examiner.

